It is well-known that information recording mediums for optical information recording or information reading include optical discs such as CD (compact disc) and DVD (digital versatile disc). A various optical discs such as read-only optical discs, recordable optical discs in which the post recording of information is possible, and rewritable optical discs on which the deleting and re-recording of information is possible have been developed.
Research and development into optical pickup devices and information recording and reading devices has proceeded alongside the high density and increased capacity of these optical discs (for example, see non-patent document 1). As a means for contending with optical discs of high density as described above, the irradiation of a light beam of small irradiation diameter on the optical disc by increasing the numerical aperture (hereinafter referred to as “NA”) of an objective lens provided in the optical pickup device has been considered. As an additional means to contend with this increased densification, utilization of a short-wavelength light beam, or more particularly a light beam produced by a blue semiconductor laser, has been examined.
However, there is a problem inherent to the increasing of the NA of the objective lens and the utilization of the short-wavelength light beam in that there is an accompanying increase in the influence of aberration on the light beam produced by the optical disc and, accordingly, the improvement of information recording and information reading accuracy becomes difficult.
More specifically, if the optical disc tilts during the process of information recording or information reading and the incident angle of the light tilts with respect to the normal direction of the optical disc (so called tilt angle), the comma aberration effect increases, In addition, although the extent of aberration changes depending on a thickness of the optical disc substrate, there are problems inherent to the reading of a CD using an optical pickup device for the reading of a DVD in which the optical disc thickness of the CD is greater than that of the DVD in that this results in the generation of spherical aberration and a significant expansion of the spot diameter of the light.
On the other hand, in high-density optical discs such as DVD that have been developed in recent years, much attention has focused on the use of a liquid crystal device as an aberration correcting element of the optical system. The liquid crystal element, which is inserted into the optical path of an optical pickup comprising a laser light source and an objective lens, corrects the phase turbulence, that is to say, the light wavefront turbulence that is caused by comma aberration that has its origin in the tilting of the optical disc substrate and by the wavefront aberration generated when a multilayer disc substrate is read. This constitutes a method in which the phase is controlled by changing of the voltage applied to the liquid crystal.
By way of example of an liquid crystal device, as shown in FIG. 1, a liquid crystal device for modulating the phase of a monochromatic light or a laser light has been disclosed in which a pair of opposing transparent substrates on which an ITO layer which constitutes an oxide of indium and tin (hereinafter referred to simply as “ITO”.) and an alignment film layer are laminated in sequence, a liquid crystal layer is provided between the alignment film layers, and an insulating film is formed between the alignment film layer and the ITO layer on at least one of the transparent substrates, and in which the relationship between the film thickness of the layers and the refractive indices thereof is optimized to minimize the light transmittance fluctuations of the liquid crystal device (for example, see patent document 1).
In addition, a liquid crystal device for correcting wavefront aberration generated in an optical system appropriately without effect of a ¼ wave plate arranged in the optical path that leads from a light source through a recording medium to a light detector has been disclosed (for example, see patent document 2). Although the technique disclosed for the liquid crystal device of patent document 2 involves the use of a curved liquid crystal substrate, this curved shape can in no way be regarded as contributing to improving the light transmittance.
[non-patent document 1] Journal of the Japan Society of Mechanical Engineers 2001.4 Vol, 104 No, 989
[patent document 1] Japanese Laid-Open Patent Publication No. 2002-208158
[patent document 2] Japanese Laid-Open Patent Publication No. 2002-251774